Chemical-mechanical polishing (“CMP”) processes are used in the manufacturing of microelectronic devices to form flat surfaces on semiconductor wafers, field emission displays, and many other microelectronic substrates. For example, the manufacture of semiconductor devices generally involves the formation of various process layers, selective removal or patterning of portions of those layers, and deposition of yet additional process layers above the surface of a semiconducting substrate to form a semiconductor wafer. The process layers can include, by way of example, insulation layers, gate oxide layers, conductive layers, layers of metal or glass, etc. In certain steps of the wafer fabrication process, the uppermost surface of the process layers are desirably planar, i.e., flat, for the deposition of subsequent layers. CMP is used to planarize process layers wherein a deposited material, such as a conductive or insulating material, is polished to planarize the wafer for subsequent process steps.
In a typical CMP process, a wafer is mounted upside down on a carrier in a CMP tool. A force pushes the carrier and the wafer downward toward a polishing pad. The carrier and the wafer are rotated above the rotating polishing pad on the CMP tool's polishing table. A polishing composition (also referred to as a polishing slurry) generally is introduced between the rotating wafer and the rotating polishing pad during the polishing process. The polishing composition typically contains one or more chemicals that interact with or dissolve portions of the uppermost wafer layer(s) and one or more abrasive materials that physically remove portions of the layer(s). The wafer and the polishing pad can be rotated in the same direction or in opposite directions, whichever is desirable for the particular polishing process being carried out. The carrier also can oscillate across the polishing pad on the polishing table.
In polishing the surface of a wafer, it is often advantageous to monitor the polishing process in situ with an end-point detection (EPD) system, e.g., to determine when a desired degree of planarization has been attained. One method of monitoring the polishing process in situ involves the use of a polishing pad having a light-transmitting region, such as an aperture or window, having translucency to light. This light-transmitting region provides a portal through which light can pass to allow the inspection of the wafer surface during the polishing process. The light-transmitting region must have sufficient light transmittance at one or more wavelengths in order for light to pass through the light-transmitting region and be detected by the EPD system.
Polishing pads having light-transmitting regions, such as apertures and windows, are known and have been used to polish substrates, such as the surfaces of semiconductor devices. For example, U.S. Pat. No. 7,614,933 discloses a polishing pad comprising a window that can be made of a rigid crystalline material, such as quartz or glass, or a softer polymeric (plastic) material, such as polyurethane. When the light-transmitting region contacts the substrate that is to be polished, polymeric materials are especially preferred so as to prevent problems that could occur when a harder window material (e.g., glass) contacts the substrate, such as, e.g., scratching of the substrate and/or light-transmitting region. The polishing pad typically is made of a polymeric material that can be the same or different from the polymeric material comprising the light-transmitting region.
Conventional EPD systems typically utilize light having a wavelength in the range of 400-690 nm, roughly corresponding to light in the visible spectrum. Newer EPD systems boast higher accuracy by employing white light comprised of both ultraviolet and visible components (e.g., about 300 nm to 800 nm), such as the FULLVISION in situ endpoint detection (EPD) system available from Applied Materials, Inc. However, conventional soft polymeric materials typically used in CMP polishing pads have poor light transmitting properties in the ultraviolet range. Moreover, these conventional materials also are highly susceptible to degradation by ultraviolet light, such that yellowing and/or brittleness can occur over time. For example, when a polishing pad comprised of a conventional polyurethane is exposed to ultraviolet light, the polyurethane will gradually degrade and crosslink, causing the polyurethane to yellow. Even ambient light can be sufficient to cause yellowing of these materials, such that special precautions must be observed when handling and/or storing polishing pads comprising conventional polymeric materials. Yellowing of the polymeric materials that comprise the light-transmitting region of a polishing pad can be especially detrimental to the functioning of EPD systems that utilize light, since these EPD systems rely on precisely monitoring changes in the wavelength and/or intensity of light passing through the light-transmitting region. In this respect, any yellowing or color change of the light-transmitting region can complicate accurate analysis of the detected light, thereby requiring, for example, frequent recalibration of the EPD system and/or replacement of the degraded polishing pad with a new polishing pad, thereby adding to the overall production time and costs.
Thus, there remains a need in the art for improved polishing pads for use in EPD systems, which polishing pads comprise, inter alia, a polymeric light-transmitting region having sufficient white (i.e., ultraviolet and visible) light-transmittance and improved stability to ultraviolet light.